Electrolytic cells for the production of aluminium comprise an electrolytic tank having a cathode and an anode generally made up of a plurality of prebaked carbon blocks. Aluminium oxide is supplied to a cryolite bath in which the aluminium oxide is dissolved. During the electrolytic processes, aluminium is produced at the cathode and forms a molten aluminium layer on the bottom of the electrolytic tank with the cryolite bath floating on the top of the aluminium layer. Oxygen is produced at the anodes causing their consumption by producing carbon monoxide and carbon dioxide gas. The operating temperature of the cryolite bath is normally in the range of 930° C. to about 970° C.
The electrolytic tank consists of an outer steel shell having carbon cathode blocks sitting on top of a layer of insulation and refractory material along the bottom of the tank. These carbon cathode blocks are connected to electrical bus bars by way of collector bars and aluminium flexibles. While the precise structure of the side walls varies, a lining comprising a combination of carbon blocks and refractory material is provided against the steel shell.
During operation of the electrolytic cell, a crust or ledge of frozen bath forms on the side walls of the electrolytic tank. While the thickness of this layer may vary during operation of the cell, the formation of this crust is critical to the operation of the cell. If the crust becomes too thick, it will affect the operation of the cell as the crust will grow on the cathode and disturb the cathodic current distribution affecting the magnetic field. On the other hand, if the frozen bath layer becomes too thin or is absent in some places, the electrolytic bath will attack the side wall lining of the electrolytic tank, ultimately resulting in failure of the side wall lining. If the attack on the side wall lining gets to the extent of the bath attacking the steel shell side walls, then the electrolytic cell has to be shut down due to the risk of metal and bath running out of the cell.
Thus controlled ledge formation is essential for good pot operation and long lifetime of the refractory lining within the cell. Furthermore, controlling the thermodynamic operation of the cell and in particular, the flow of heat from the bath through the side wall lining is essential for controlled ledge formation within the cell.
In recent technology developments, heat is removed from the cell through the steel shell of the electrolytic tank using passive heat transfer devices such as radiating fins in an attempt to increase the surface area available for heat transfer from the side walls of the electrolytic tank. The heat needing to be removed from the electrolytic cell is dependent upon the amount of current passing through the cell and the cell voltage. If there is an increase in the current or voltage, then the heat which needs to be extracted through the side wall to maintain an appropriate thickness of ledge formed on the inner wall of the refractory material will increase and can often vary beyond the design capabilities of the passive cooling elements on the side of the electrolytic cell.
Accordingly, it is an object of the present invention to provide a means by which the thermodynamic requirements of an electrolytic cell can be actively controlled to enable the formation and maintenance of a ledge on the inner surface of the side wall refractory material.